Cellular systems simultaneously handling a number of traffic channels through each base station are known. Such systems are typically assigned a number of channels (f.sub.1 -f.sub.n) in support of communications with mobile communication units through such local base stations. Each base station is, in turn, allotted a subset of the channels (f.sub.1 -f.sub.n). Of the subset of channels assigned to a base site at least one (and often more) is designated as a control channel for purposes of access control and channel set-up.
Communication with a communication unit on a traffic channel within a service coverage area of the base site is often accomplished through an omnidirectional antenna centrally located within the service coverage area. A number of communications transactions may be simultaneously supported through the antenna with each individual communication supported by a transmitter (located at the base site) assigned to the traffic channel. Each transmitter includes a modulated transmit signal source within the transceiver and a radio frequency (RF) power amplifier). Each transmitter thereby provides signal generation, modulation and amplification.
The simultaneous transmission of a number of traffic channel signals from the central antenna requires that the transmitter output of each active transceiver be combined before application to, and transmission from, the central antenna. In order to avoid interference-producing intermodulation products, signals must be combined after any non-linear steps within the amplification process. In addition, the combining topology must provide sufficient reverse isolation to insure that signals of parallel amplification branches will not be coupled into the output of other power amplifiers, again producing intermodulation products.
Where each transceiver is equipped with its own power amplifier (PA), combining must occur after the PA where signal levels, as well as combining losses, are high. A cavity combiner, for combining such high level RF signals while providing the necessary isolation, is provided by U.S. Pat. No. 4,667,172 assigned to the assignee of the present invention.
In other communication systems, transceivers are not equipped with individual PAs; instead, a common, multitone linear PA (LPA) is used for amplification after the RF signals have been combined at relatively low power levels at the output of the transceiver. The use of such common LPA for traffic channels in systems using a common antenna has resulted in considerable simplification of system topology, improvements in system efficiency, and reduction in system size.
The use of an LPA, on the other hand, has certain disadvantages particularly where RF signals are placed on evenly spaced channel frequencies and phase locked to a common frequency source. In such a case, amplitude fluctuations resulting in signal clipping may occur where a peak envelope power of the composite signal exceeds the LPA's power rating.
FIG. 1 demonstrates the effects of signal phasing in a simplified case involving three signals, A, B, and C, during a time period T. The three signals are shown in FIGS. 1-1, 1-2, and 1-3 respectively. The envelope of the summed composite signal (absolute value of A+B+C) is shown in FIG. 1-4. As may be observed, an envelope peak occurs during the middle of the period (T/2), when all three signals are in phase. The magnitude of this peak can be reduced by reversing the phase of signal C (taking the absolute value of A+B-C), as shown in FIG. 1-5.
Clipping may occur in an LPA when the peak envelope power of the composite input signal (squared envelope magnitude), multiplied by the gain of the LPA, exceeds the peak output power capability of the LPA. Peaks resulting from phase matches have been observed to last for periods of from one to ten seconds, or longer in some systems. Clipping of the RF signal results in the generation of intermodulation products on other RF channels and degradation of system performance.
Clipping due to summation of in phase signals is most severe when the carriers of such signals are unmodulated (during speech pauses) or weakly modulated (during low energy portions of the speech waveform). Full modulation of the carriers produces random variations in the carrier phases which limits the duration of any clipping to time periods on the order of one millisecond. Where the carriers are unmodulated or weakly modulated, however, a repetitive clipping process may occur which is periodic at a rate equal to a frequency difference of contributing carriers. This repetitive clipping process is what gives rise to the generation of strong intermodulation products when the carriers are close in phase.
Past efforts to control peak envelope power due to phase summations have included de-rating of LPAs or intentionally de-correlating frequency references. Derating accommodates phase peaks by requiring an inordinately large LPA. De-correlating frequency references is effective in that where peaks do occur the peaks are very short and, consequently, more easily tolerated. De-correlating carriers, on the other hand, creates problems in synchronization not only in receiving control information on other channels but also in handoff among base sites.
The use of de-correlated (independent) frequency references is expensive and inefficient. The use of an inordinately large LPAs reduces the benefits inherent in signal combining at low power levels. A need exists for a more efficient method of peak envelope power control within a LPA.